Laser printers have been a relatively recent improvement over dot matrix and ink jet-type printers typically used with personal computers or workstations. A typical resolution in dots per inch (dpi) of an ink jet printer was 300 dpi, whereas most laser printers have an improved resolution of 600 dpi.
Further advancements in laser printers have now made it possible to manufacture such printers having an even more improved resolution of 1200 dpi. Many existing computer files, however, are not in a position to immediately take advantage of the higher resolution capabilities of such latest laser printers. Some of these computer files are word processor data files or computer-aided-drafting (CAD) data files, and were originally created to be printed at only 300 dpi or 600 dpi during a time when those lower resolution capabilities were the best that was available. A 300 dpi or 600 dpi data file can be expanded into a 1200 dpi file quite easily by halving the physical size of each of the dots (or pels) and doubling the number of such pels per unit distance. This expansion would take place in both the horizontal and vertical directions.
Unfortunately, a simple expansion of the bitmap resolution from 300 dpi to 1200 dpi, or from 600 dpi to 1200 dpi (at either a four-times or two-times expansion factor) does not result in a printed page having an appearance any better than the originally printed page at the lower resolution. In fact, the expanded printed page may have diagonal lines that actually appear to be more jagged than the original page when printed using a lower resolution printer, because the higher resolution laser printer can more accurately place each of the dots on the printed page such that the expanded resolution printed page will be even more accurate and have a sharper line delineation than the original printed page. This would be true whether the bitmap data represented alphanumeric data (i.e., letters and numbers) or drawing data created by a CAD or other graphical data system.
The need for a "smooth" expanded bitmap image has been addressed in the prior art, particularly by U.S. Pat. No. 4,437,122 (Walsh et al.). Walsh enhances the resolution quality of alphanumeric characters for a system that receives bitmap information initially in the form of pixels for a video display, then provides a hard copy output (via a printer). Walsh analyzed successive lines of video data by comparing pixels on three successive lines that surround each specific individual pixel to be analyzed, to determine if any of the edges of the specific pixel being analyzed should be rounded off or have its diagonal edges smoothed. After the analysis of the individual pixel has occurred, the apparatus of Walsh generates an identifier that uniquely describes the isolated pixel pattern (i.e., the pattern surrounding the specific pixel of interest), then compares the identifier with standard pixel formats that are already stored in a memory in the form of a look-up table, then enhances the pixel of interest by replacing it with the appropriate standard pixel format (from the look-up table).
While the Walsh apparatus is an improvement over a completely unsmooth expanded bitmap, the processing time required to analyze the pixel pattern around the particular pixel of interest could greatly slow down the printing process for a laser printer, and also requires a look-up table for each font desired to be used in a particular laser printer system. Furthermore, it would not be readily usable for graphical data in the form of drawings, such as those generated by a CAD system.